1. Field of Invention
The present invention is intended to transmit ions from higher to lower pressure regions such as atmospheric pressure interfacing of ionization sources to chambers containing mass spectrometers or ion mobility spectrometers.
2. Description of Prior Art
Dispersive sources of ions at or near atmospheric pressure; such as, atmospheric pressure discharge ionization, chemical ionization, photoionization, or matrix assisted laser desorption ionization, glow discharge, and electrospray ionization generally have low sampling efficiency through conductance or transmission apertures, where less than 1% [often less than 1 ion in 10,000] of the ion current emanating from the ion source make it into the lower pressure regions of the present commercial interfaces for mass spectrometry.
U.S. Pat. No. 4,542,293 to Fenn, et al. (1985) demonstrated the utility of utilizing a dielectric capillary, a glass tube, to transport gas-phase ions from atmospheric pressure to low pressure where the viscous forces within a capillary push the ions against a potential gradient. This technology has the significant benefit of allowing grounded needles with electrospray sources. Unfortunately, this mainstream commercial technology transmits only a fraction of a percent of typical atmospheric pressure generated ions into the vacuum. The majority of ions being lost at the inlet due to dispersive fields dominating the motions of ions (see FIG. 8). The requirement of capacitive charging of the tube for stable transmission, as well as, transmission being highly dependent on surface charging creates limitations on efficiencies with this technology. Contamination from condensation, ion deposition, and particulate materials can change the surface properties and the transmission. Because of the large surface area contained on the inner wall surface, a large amount of energy is stored and can discharge and damage the electrode surfaces. Care must also be taken to keep the outer surfaces clean and unobstructed, presumably in order not to deplete the image current that flows on the outer surface of the glass tube.
U.S. Pat. No. 4,977,320 to Chowdhury, et al. (1990) demonstrated the use of heated metal capillaries to both generate and transmit ions into a vacuum chamber. The efficiencies of this device are low as well. This technology samples both ions and charged droplets into the capillary where, with the addition of heat, ion desorption is facilitated. Undergoing coulomb explosions inside the restricted volume of the tube will tend to cause gas-phase ions to be loss to the walls of the tube due to dispersion. In addition, this technique encounters the same limitation from dispersion losses at the inlet as the dielectric capillaries.
Lin and Sunner (1994) (J. American Society of Mass Spectrometry, Vol. 5, Number 10, pp. 873–885, October 1994) studied a variety of effects on transmission through tubes of glass, metal, and Teflon. A wide variety of parameters were studied including capillary length, gas throughput, capillary diameter, and ion residence time. Effects from space charge, diffusion, gas flow, turbulence, spacing, and temperature where evaluated. These studies failed to identify the field dispersion at the inlet as the major loss mechanism for ions in capillaries. Some important insights where reported with respect to general transmission characteristics of capillary inlets.
U.S. Pat. No. 5,736,740 to Franzen (1998) proposed the use of weakly conducting inner surfaces to prevent charge accumulation as a means to facilitate the focusing of ions toward the axis of the capillary. Although it is difficult to distinguish this art from Fenn in that the glass tubes utilized in commercial applications under Fenn also utilize weakly conducting dielectric surfaces, Franzen does argue effectively for the need to control the inner surface properties and the internal electric fields. This device will suffer from the same limitations as Fenn.
U.S. Pat. No. 5,747,799 to Franzen (1998) also proposed the need to focus ions at the inlet of capillaries and apertures in order enhance collection efficiencies. In this device the ions are said to be entrained into the flow by viscous friction. This invention also fails to account for the dominance of the electric field on the motion of ions in the entrance region. At typical flow velocities at the entrance of tubes or apertures, the electric fields will dominate the ion motion and the ions that are not near the capillary axis will tend to disperse and be lost on the walls of the capillary or aperture inlet. With this device, a higher ion population can be presented to the conductance opening at the expense of higher field ratios and higher dispersion losses inside the tube.
WO Patent 03/010794 A2 to Forssmann, et al. (2002) utilized funnel optics in front of an electrospray source in order to concentrate ions on an axis of flow by imposing focusing electrodes of higher electrical potential than the bottom of the so called accelerator device. This device frankly will not work. The ions formed by the electrospray process will be repelled by this optics configuration and little to no transmission will occur. Most of the inertial energy acquired by the ions in the source region is lost to collisions with neutral gas molecules at atmospheric pressure; consequently the only energy driving the ions in the direction of the conductance aperture will be the gas flow which under normal gas flows would be insufficient to push the ions up a field gradient. This device does not operate in fully developed flow as will be described in the present invention.
U.S. Pat. No. 6,359,275 B1 to Fischer, et al. (2002) addressed the issue of charging of the inner surface of the capillary by coating the inner surface with a conductor in the dispersive region of the tube while still keeping the benefits of the dielectric tube transport in the nondispersive region of the capillary. This approach addresses the problem of charge accumulation, but it does not remove the significant losses due to dispersion at the inlet (as shown in FIG. 8).
U.S. Pat. No. 6,486,469 B1 to Fischer, et al. (2002) utilized external electrodes and butted capillary tubes to provide enhanced control of the electric field within the capillary. This device does not (as pointed out in the preceding paragraphs) address issues related to inlet losses as presented in FIG. 8. In addition, the device still required significantly large dielectric surfaces with the associated problems of charging and contamination of the inner tube surfaces, and potential for a discharge.
U.S. Patent Application US 2003/003452 A1 and U.S. Pat. No. 6,583,407 B1 both to Fischer, et al. (2003) utilized a variety of modifications to their dielectric tube device to enhance selectivity and control of ions as they traverse their capillary device. None of these modifications addresses the aforementioned limitations of capillaries or tubes; namely loses at the tube inlet, charging and contamination of the inner tube surfaces, and potential discharge.
U.S. Pat. No. 6,455,846 B1 to Prior et al. (2002) disclosed a flared or horn inlet for introducing ions from an atmospheric ionization chamber into the vacuum chamber of a mass spectrometer. They reported that the increase in ion current recorded in the mass spectrometer was directly proportional to the increase in the opening of the flared inlet.
U.S. Pat. No. 6,897,437 to Fuhrer et al. (2005) disclosed a microchannel plate as a pressure drop and ion interface between an ion mobility spectrometer or ion drift cell at higher pressure and a chamber at lower pressure where a mass spectrometer resided. They proposed that ions would be transferred from this higher pressure region into the lower pressure region by reverse biasing the semi-conductive capillaries in the presence of gas flow and a temperature gradient as described in U.S. Pat. No. 5,736,740 to Franzen (1998), see above. No experimental results were disclosed.
U.S. Pat. No. 6,583,408 B2 to Smith et al. (2003) has recently utilized multi-capillary arrays as an inlet to their ion funnel technology. This device reports an advantage of bundle tubes over single opening conductance pathways, but fails to address the major issue relating to ion transmission loss, namely field dispersion of ions at the entrance of the conductance opening. As described above with a single tube, without controlling the field throughout the conductance path a bundle of tubes will still have significant losses when sampling higher electrical field sources.
Ion movement at higher pressures is not governed by the ion-optical laws used to describe the movement of ions at lower pressures. At lower pressures, the mass of the ions and the influence of inertia on their movement play a prominent role. While at higher pressures the migration of ions in an electrical field is constantly impeded by collisions with the gas molecules (˜2 million collisions per millimeter). In essence at atmospheric pressure there is so many collisions that the ions have no “memory” of previous collisions and the initial energy of the ion is “forgotten”. Their movement is determined by the direction of the electrical field lines and the viscous flow of gases. At low viscous gas flows, the ions follow the electric field lines, while at higher viscous gas flows the movement is in the direction of the gas flow. We have previously disclosed various means of moving ions at atmospheric pressure by shaping the electric field lines and directing the flow of gases. FIG. 8 illustrates a simulation of ion trajectories under the forces of both electric field and gas flow. Experimental evidence and theory support the premise that the electric field dominates the motion of ions at the entrance region of apertures and tubes of most high field sources where ions are focused at the conductance opening.
Nevertheless inlet apertures, be they tubes or pin hole apertures, heretofore known suffer from a number of disadvantages:
(a) If one uses a tube or capillary inlet to a lower pressure chamber there are loses at the entrance, along the length of the tube, and at the exit. Our U.S. Pat. No. 6,943,347 (2005) describes the use of laminated tubes to control both field and flow throughout the entire conductance pathway; from the entrance to the exit. Delaying dispersion of the ions until flow has fully developed is described in this patent as a technique to minimize dispersion losses within the conductance pathway. FIGS. 9 and 10 of U.S. Pat. No. 6,943,347 illustrated the typical flow development within a laminar flow tube and the lack of dispersion when laminated tubes are utilized to maintain uniform field throughout the tube, respectively. The principals and methods of this patent are applied to the present invention where our laminated arrays operate with the same ion transmission advantage as observed with laminate tubes.
(b) Inlet tubes made of glass offer poor transmission efficiency of gas-phase ions due to (1) loss of ions at the inlet due to the dispersive electric fields, (2) the requirement of capacitive charging of the tube leads to slow and unpredictable start-up conditions, and (3) contamination of the inner surfaces of the tube changes the capacitance of the tube leading to unpredictable transmission and possible initiation of a discharge inside the tube.
(c) Inlet tubes made of metal fair no better-loss of ions at the tube entrance and along the length of the inner surfaces due to dispersive electrical fields.
(d) Pin hole apertures also suffer from the loss of ions at the entrance due to the dispersive electrical fields.
(e) Inlet tubes made of glass are very fragile, thereby requiring special handling and storage; and the need for precautions while removing and installing the tubes.
(f) The use of arrays of inlet tubes does increase the surface area of the inlet. It does not eliminate the dispersive electrical fields present at the entrance of each individual inlet but actually multiplies the effect over a larger area.
(g) Inlet apertures made of silica or doped glass, such as microchannel plates (MCP) are generally not resistant to commonly used solvent or salts used in liquid chromatography (LC). The dielectric or silica surfaces are composed primarily of an alkali doped silica layer, and as a result, these materials are very hydroscopic (see Burle Industires, Inc. Lancaster, Pa., USA, www.burle.com). Special care and handling is necessary in order to prevent the absorption of water vapor and other components to ensure optimal performance. For example, prior to use microchannel plates are typically stored in a foil bag that has been back-filled with dry nitrogen and then evacuated. If not used, the MCP should be removed from its shipping case and the foil bag and stored in a vacuum or if vacuum storage is unavailable, storage in dry nitrogen is a suitable alternative. In addition, MCP can degrade by exposure to various types of hydrocarbon materials (such as alcohols, a common LC liquid phase) which raises the work function of the surface—causing the insulating silica layer's dielectric properties to change over time, charging of the surfaces leading to the formation of a repulsive barrier, initiating a discharge, etc. Operation in a clean vacuum environment of 10−5 torr or better is necessary in order to ensure the long-life characteristics of these devices. In addition, exposure to water can lead to swelling of the silica layers and eventually cracking or shattering of the MCP inlet; leading to a unpredictable catastrophic venting of the vacuum chamber. Damaging vital components of the analyzer, such as electronics, vacuum pumps, etc.
3. Objects and Advantages
The objective of the present invention is to maximize the transmission of ions from one pressure regime into an adjacent lower pressure region through an array of apertures in a laminated lens while minimizing the conductance of gas from the higher pressure into the lower pressure region. The relatively uniform electrostatic field through the laminated lens assures high transmission and low dispersion of the ions while in the conductance pathways of the lens. This condition does not exist in present-day ion conductance pathways in atmospheric or high pressure interfaces for mass spectrometers and will result in significantly higher ion transmission through conductance paths compared to the current art. Several objects and advantages of the present invention are outlined below.
(a) To provide an ion enrichment aperture that imparts a lower gas load on lower pressure regions while maintaining the transmission of ions. This has beneficial implications including lower requirements for pumping, power, and general size. Conversely, this device has higher transmission of ions for a given total gas load on the lower pressure region resulting in more sensitive response for ion analyzers or higher currents for current deposition processes. Utilizing small apertures in the arrays results in very low electrostatic field penetration into the lower pressure region relative to larger apertures with higher conductance.
(b) To provide an ion enrichment aperture whose array of openings is matched to the conductance pattern. A macroscopic lens can be patterned to focus the ions to a microscopic compressed pattern of conductance openings. In other words, with patterned arrays we can focus the ions to an exact pattern of conductance openings rather than being required to focus to a single opening of a tube or aperture.
(c) To provide an ion enrichment aperture with the ability to measure the transmission of ions in discrete packets, each representative of a portion of the delivered cross-section from a source of ions. With this capability we are able to independently measure each pathway to discern the cross-section composition of a source of ions. This increased information content adds an enhance dimension to analysis where composition across a cross-section may provide diagnostic, feedback, or analytical information.
(d) To provide an ion enrichment aperture with the ability to be heated and measure the temperature of the aperture. With this capability we are able to provide feedback control of the temperature of the aperture, maintaining the aperture at a prescribed temperature.
(e) To provide an ion enrichment aperture with the ability to introduce gas into the individual openings of the aperture. The gas flowing out of the openings towards the ion source, gas flowing counter to the movement of the ions, and gas flowing with the movement of the ions into the lower pressure region. With this capability we are able to use viscous forces to focus ions towards the axis of the individual openings as they enter the openings and to move ions into the lower regions. Minimizing ions impacting or depositing and contaminating the interior surfaces of the apertures, the inlet and outlet laminates; and the associated reduction in required maintenance, system drift, charging, and potential carryover from sample to sample due to deposition.
(f) To provide an ion enrichment aperture which facilitates higher transmission of ions from any number of pressure regimes, including above atmospheric pressure, atmospheric pressure, and intermediate pressures. There may be practical uses of this approach even in the millitorr region, although inertial components of motion and scattering will begin to degrade performance below about one torr.
(g) To provide an ion enrichment aperture which can be used for transmitting ions from higher pressure ion sources into lower pressure destinations. Examples of ionization sources operating at high pressures would be atmospheric pressure or intermediate pressure sources, such as but not limited to, electrospray (ES), atmospheric pressure chemical (APCI) and photoionization (APPI), inductively coupled plasmas (ICP), MALDI (both atmospheric pressure and reduced pressures), glow-discharge, RF and or DC discharges, etc. Examples of lower pressure destinations would be ion analyzers, such as but not limited to, mass spectrometers or ion mobility spectrometers, surfaces in vacuum where the deposition of thin films and etching processes are preformed, etc.
(h) To provide an ion enrichment aperture which can be produced in a variety of sizes, patterns, shapes, and materials.
(i) To provide an ion enrichment aperture which can be fabricated with a wide variety of fabrication alternatives not presently used with apertures, such as but not limited to, micro-machining; microlithography for creating patterns; etching for removing material; depositing techniques such as ion beam deposition, screen print, stenciling; printed circuit board fabrication, etc.
(j) To provide an ion enrichment aperture whose production allows for a convenient, fast, and economical change of the number of laminates, number of openings, and thickness of the individual laminates that are being produced.
Further objects and advantages are to provide an ion enrichment aperture which can be used easily and conveniently incorporated into existing atmospheric interfaces without the need for extensive or major reconstruction, which is simple to use and inexpensive to manufacture, which can be produced en masse or separately, which can be retrofitted to existing vacuum chambers or designed specifically for new vacuum chamber, which can be used with either highly dispersive, or low electrostatic or electrodynamic field ion sources, which can be used repeatedly, which is resistant to common gases and solvents used with the techniques of mass spectrometry, such as but not limited to, liquid chromatography mass spectrometry (LC/IMS), and which obviates the need for precise alignment of the ion source relative to the aperture. Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings.